1. Field of the Invention
The present invention relates to a telephoto zoom lens, and more particularly, to an inner focusing type fast supertelephoto zoom lens in which part of lens groups in the lens system is movable for focusing.
2. Related Background Art
Automation of various control devices for use in a camera has taken place in recent years. Single-lens reflex cameras with an autofocusing (hereinafter referred to as an `AF`) device provided therein have been available. Autofocusing of fast supertelephoto zoom lenses for use in such an AF camera has encountered the following difficulties.
The telephoto zoom lenses of the above-described type are long, and the distal end portion thereof is heavy. Conventionally, such telephoto zoom lenses are mainly of the front focusing type in which a lens group having a large diameter and located closest to an object is moved as a lens group which is movable along an optical axis for focusing (hereinafter referred to as a `focusing lens group`).
Although this front focusing method has an advantage in that it can be accomplished by a simple lens configuration, the focal length of the front lens group which is movable for focusing is very long, and the distance through which the front lens group is moved for focusing is thus large. Furthermore, as the aperture ratio increases, the outer diameter of the zoom lens increases, thereby increasing the weight thereof. Therefore, in the front focusing method, the workload, i.e., the load, of the driving device for accomplishing autofocusing is greatly increased. This hinders autofocusing of the above-described type of lens.
The front focusing method also suffers from problems in that the marginal lumination is decreased and the center of gravity of the lens is shifted due to changes in the overall length of the zoom lens caused by the movement of the front lens group movable for focusing, which is conducted at a close focusing distance, and in that the shortest object distance cannot be reduced sufficiently.
In order to overcome these problems, it has been proposed to construct the lens portion located closer to the object than a lens group which is movable for changing magnification (hereinafter referred to as `a magnification changing lens group`) using a plurality of lens groups and to make movable for focusing the lens group located farthest from the object in these plurality of lens groups or part of the lens groups located in the intermediate portion of these plurality of lens groups. Such inner focusing type telephoto zoom lenses have been proposed in, for example, (a) Japanese Patent Publication Nos. 59-4688 and 62-53696, (b) Japanese Patent Laid-Open No. 63-188110, and (c) Japanese Patent Laid-Open No. 59-23314.
However, the zoom lenses classified as (a) to (c) have the following problems.
In the zoom lenses itemized in (a), since the diameter of the focusing lens group disposed farthest from the object is substantially the same as that of the lens group having the large diameter and located closest to the object and thus is not very small, the weight of the focusing lens group is increased. Furthermore, since the refractive power of the focusing lens group is weak, the movement of the lens group for focusing is increased. Consequently, the load of the driving device is increased, and the size of the lens barrel is also increased.
In the zoom lens itemized in (b), although the diameter of the focusing lens group disposed farthest from the object is slightly smaller than that of the zoom lens itemized in (a), it is not small enough to decrease the load of the driving device. Furthermore, when the zooming ratio of the zoom lens is increased by enlarging the magnification changeable range toward the wideangle side, the image quality deteriorates when the zoom lens is used at the telephoto end due to the curvature of image of color (g line) which occurs at the edge of the image field.
In the zoom lens itemized in (c), the focusing lens group is located between the fixed lens disposed closest to the object and the other fixed lens disposed at the rear of that fixed lens, and has small diameter and large refractive power. Therefore, both the weight and movement of the focusing lens group are small. However, since the aperture stop is located between the focusing lens group and the other fixed lens located at the rear of the fixed lens disposed closest to the object, the exit pupil of the zoom lens used at the wideangle end is shifted from the image field through a large distance. Consequently, in order to provide an amount of necessary light, the diameter of the lens group located closest to the image field is increased, and enlargement of the magnification changeable range toward the wideangle end is difficult. This in turn makes an increase in the zooming ratio difficult. Furthermore, the accuracy of the distance detection system in the AF device deteriorates.
Thus, in the telephoto zoom lenses disclosed in (a) to (c), the compact form and sufficient degree of autofocusing are not compatible.